It is already known that a fan jet type nozzle at one atmosphere of pressure will give about 45% of its number of droplets under 30 .mu.m, and consequently when open spraying in agriculture, there is a general wastage of spray chemicals used for example for weedicide applications due to the droplets becoming airborne. Many weedicides are exceedingly effective, and for example the weedicide sold by Monsanto under the registered trade mark "ROUNDUP" can be effective when used in very small quantities. Hence overspray can be very damaging. However, it is known that droplets which exceed 400 .mu.m will frequently bounce off foliage, and be wasted on the ground. For this reason, there has been much development into rotational spray devices which can give selective narrow ranges of droplet size from 50 to 300 microns, and wherein about 90% of the droplets fall in any one selected range (for example a range of 70 to 130 micron).
The British company CDA Ltd, of Lockinge, Wantage, Oxon, UK, distributed a pamphlet in October 1983 entitled "The Principles of CDA Spraying." Experimentation by the inventor herein has generally confirmed the information contained in that pamphlet to be the most accurate available to him. It compared conventional nozzle spraying with so-called "constant droplet application" (CDA) spraying which utilised a micro,spray head having spinner to produce droplets, and illustrated that a "conventional nozzle" which was tested, distributed some chemical in small droplets below 40 microns size (which was both wasted and damaging due to `overspray`), and about one half the chemical was in droplets exceeding 150 microns, but included in a very small number of droplets.
This compared very unfavourably with a Constant Drop Applicator utilising a spinner, wherein only a small number of droplets were less than 35 microns, and none less than 15 microns, and none exceeded about 120 micron size. Most of the droplets were between 70 and 110 micron size, and accounted also for most of the chemical. Thus wastage was largely avoided, which would otherwise have been due to overspray of small droplets, and excessive quantities of chemical in large droplets.
The main object of this invention is to further enhance the advantages of using a CDA sprayer, and, if necessary enhance the use of a conventional sprayer, in both cases reducing wastage of chemicals due to wind losses or large droplets.
It is already known to provide a towable spraying device for attachment to a tractor to reduce the drift of finely misted particles of weedicide, and one example can be seen in the U.S. Pat. No. 4,186,879 in the name of KINDER, wherein a generally rectilinear shroud was provided with depending flexible baffles to comprise fore and aft wall sections of the shroud and engage and ride over rough terrain while still confining the spray within the shroud.
However, a sprayer must essentially be used under terrain conditions which vary widely. A rectilinear shroud must not be permitted to encounter such obstructions as a tree trunk or vine stem, since crop damage will otherwise ensue.
As far as is known to the applicant, a satisfactory provision has not heretofore been forthcoming to enable a towable device to spray weeds close to the stems of vines, bushes or trunks of trees, without endangering leaves, trunk or stem, and for example in the substantially rigid structure previously proposed, if carried too close to the trees or vines, there is a danger of the trunks or stems being damaged. In an extreme case, a sudden lurch due to engagement of the towed or towing vehicle with the walls of a plough furrow could conceivably result in destruction of a young plant.
The main object of this invention therefore is to provide improvements whereby it is possible to spray close to the stems of the vines, bushes or trees without endangering them either due to overspray reaching the leaves or in some instances reaching the green stalks or stems of the bushes or vines. It is a further object to inhibit loss due to wind drift so that a very fine mist of spray can be applied in minute quantities and yet be effective.